210 ANALYTICAL MECHANICS The three types of equilibrium are illustrated by the three equilibrium positions, Fig. 106, which a right cone can assume on a horizontal plane. FIG. 106. NEWTONIAN POTENTIAL. 175. Newtonian Law of Force. — The law of force between two interacting particles is called a Newtonian law of force if the particles attract or repel each other with a force which acts along the line of centers of the particles, and which varies directly as the product of the masses of the particles and inversely as the square of the distance between them. The forces between two material particles, between two small electrical charges, and between two small magnetic poles obey the Newtonian law of force. The following are the familiar forms in which the law is written for material, electrical, and magnetic particles, respectively, where 7, k, and /* are constants. When the interacting particles are in free space the numerical value of the constants k and p, is unity, while Y=6.7X10-8 gm. sec.2 176. Newtonian Field of Force. — When the forces which act in a region obey the Newtonian law of force the region :s called a Newtonian field of force.